1. Field of the Invention
The present invention relates to a liquid droplet ejection head and an image forming apparatus, and more particularly to a liquid droplet ejection head and an image forming apparatus which each eject a functional liquid containing dispersed microparticles.
2. Description of the Related Art
An inkjet printer (inkjet recording apparatus) is known as an image forming apparatus. An inkjet printer comprises an inkjet head (liquid droplet ejection head) in which a large number of nozzles (liquid droplet ejection ports) is arranged, and records an image or pattern on a recording medium by ejecting ink in the form of liquid droplets through the nozzles toward the recording medium while moving the inkjet head and the recording medium relatively to each other.
For example, an inkjet head of such an inkjet printer comprises a pressure chamber unit including: a pressure chamber which is supplied with ink from an ink tank via an ink supply passage (common liquid chamber); a piezoelectric element which is driven by electric signals corresponding to image data; a diaphragm constituting a part of the pressure chamber, which is deformed by driving the piezoelectric element; and a nozzle which communicates with the pressure chamber and ejects the ink in the pressure chamber in the form of liquid droplets when the volume of the pressure chamber decreases due to the deformation of the diaphragm. By combining dots formed by the ink that is ejected through the nozzles of the pressure generating units in the inkjet printer, an image or pattern is formed on a recording medium.
In recent years, inkjet printers have been used not only to form images by depositing a liquid such as ink on a sheet of recording paper, but also to form patterns and the like by ejecting a functional liquid containing dispersed microparticles. Embodiments of the microparticles that may be used include pigment, polymer resin, metal, glass, and oxides or compounds thereof.
When a raw material such as those described above is formed into microparticles and the microparticles are then dispersed and emulsified into a solvent liquid using a dispersing agent, the microparticles in the liquid aggregate and settle over time. If a functional liquid containing these aggregated and settled microparticles is ejected as is, unevenness occurs in the concentration or microparticle density of the liquid, leading to concentration unevenness or distortion in the ejection result and deterioration of the color reproducibility. As a result, it is difficult to maintain a high ejection result quality.
Under such circumstances, for example, Japanese Patent Application Publication No. 2003-72104 discloses an apparatus that maintains microparticles in a functional liquid in a consistently dispersed state. In the apparatus, a manifold (common liquid chamber) in which an ejection substance accumulates immediately before being ejected through a nozzle is provided in the interior of an ejection head. The manifold is provided with a stirring device such as a piezoelectric element, for example, and hence vibration is provided by the stirring device to the ejection substance in the manifold such that the ejection substance is stirred constantly.
As noted above, in a functional liquid containing dispersed microparticles, the microparticles in the liquid aggregate and settle over time. Particularly when the functional liquid is an adjusted ink in which a raw color material formed into microparticles is dispersed through a solvent liquid, the color material in the ink aggregates and settles over time such that the ink concentration is greater in a lower layer portion than an upper layer portion. When such ink is ejected by an ejection head structured such that the ink is supplied to a pressure chamber through a supply passage from the lower portion of a common liquid chamber, the thick ink that has settled near the bottom portion of the common liquid chamber is supplied first. As the ink is consumed by the print operation, the density of the image formed by the ejected ink gradually shifts from dark to light, and hence the image quality deteriorates.
In the apparatus disclosed in Japanese Patent Application Publication No. 2003-72104, in order to solve the problem described above by preventing the microparticles in the liquid from settling, the microparticles are maintained in a consistently dispersed state by providing the piezoelectric element as a device for stirring the ejection substance in the common liquid chamber (manifold) so that the ejection substance is subjected to vibration and stirred at all times. However, this stirring mechanism requires a drive circuit to drive the piezoelectric element, a control device to synchronize the stirring operation and ejection operation, and the like, leading to an increase in cost.